1. Field of the Invention
This invention relates to a speed adjusting apparatus for a pneumatically driven power tool, which has a tool head that can rotate in two directions for driving nuts or bolts, more particularly to a speed adjusting apparatus, which can be operated to vary the rotational direction of the tool head.
2. Description of the Related Art
Referring to FIG. 1, a conventional pneumatically driven tool 1 is shown to include an outer housing 11 with a chamber 111 and a handle 112 that is disposed under the chamber 111. A driving mechanism 12 is mounted within the chamber 111, and includes an impact device, a front bearing member, an air cylinder with a rotor, and a rear bearing member 121. The rear bearing member 121 has a forward hole 122 and a reverse hole 123, which are formed therethrough. Because the improvement of this invention is not directed to the driving mechanism 12, a more detailed description thereof will be omitted herein. To introduce air into the air cylinder, the handle 112 has an intake passageway 113, a mounting passageway 114, and a connecting passageway 110 that intercommunicates the passageways 113, 114. The intake passageway 113 intersects an elongated positioning hole 115 at an upper end thereof. For the purpose of adjusting rotational speed and controlling rotational direction, the tool 1 further includes a rear cover 14 bolted onto a rear end of the outer housing 11, a rotational-direction controlling mechanism 15 mounted within the positioning hole 115, a speed adjusting valve 16 disposed within the intake passageway 113, and an air controlling mechanism 17 mounted within the mounting passageway 114.
The rotational-direction controlling mechanism 15 includes a tubular member 151 disposed within the positioning hole 115, and a control rod 152 that is mounted movably within the tubular member 151 and that has two annular grooves 153. The tubular member 151 has an intake port 154 that is communicated with the intake passageway 113, a forward air-controlling passageway 155 and a reverse air-controlling passageway 156 that are communicated respectively with two passageways 116 (only one is shown) in the outer housing 11. The passageways 116 are communicated respectively with a forward guiding slot 141 and a reverse guiding slot 142 in the rear cover 14 so as to guide air into the forward port 122 and the reverse port 123 in the rear bearing member 121, thereby rotating the rotor clockwise or counterclockwise.
The speed adjusting valve 16 has an axial passageway 161 that is communicated with the intake passageway 113, and a plurality of radial holes 162 that are communicated with the axial passageway 161 and that have different diameters. Any of the radial holes 162 can be communicated with the connecting passageway 110 by rotating the valve 16 relative to the handle 112. The larger the diameter of the radial hole 162 communicated with the connecting passageway 110, the faster the tool head rotates. The air controlling mechanism 17 includes a sliding rod 171 that is mounted movably within the mounting passageway 114. The sliding rod 171 has an upper end that contacts a pressing plate 172, and a lower end, which contacts a ball 175 that is biased by a spring 174 and that is located under a sealing washer 173. A connector 176 is mounted threadably within a lower end of the mounting passageway 114.
To rotate the tool head counterclockwise, the control rod 152 is positioned so that one of the annular grooves 153 in the control rod 152 is communicated with the intake port 154 and the reverse air-controlling passageway 156 in the tubular member 151. Under this circumstance, upon depression of the pressing plate 172, the ball 175 moves downward to separate from the washer 173 so as to permit air flow from the connector 176 into the rotor through a flow path, which consists of the connecting passageway 110, one of the radial holes 162, the axial passageway 161, the intake passageway 113, the intake port 154, the reverse air-controlling passageway 156, the passageways 116, the reverse guiding slot 142, and the reverse port 123. The rotational direction of the tool head can be varied by moving the control rod 152 within the tubular member 151. The rotational speed of the tool head can be adjusted by rotating the valve 16 in the tool 1.
Numerous elements and passageways are provided in the rotational-direction controlling mechanism 15 and the speed adjusting valve 16, thereby resulting in a complicated structure for the tool 1. Furthermore, high precision is needed to ensure the structural relationship between the tubular member 151 and the positioning hole 115 and between the tubular member 151 and the control rod 152. Accordingly, a single apparatus is required to perform the functions of adjusting the rotational speed of the tool head and controlling the rotational direction of the tool head for simplifying the construction of the power tool.